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TPS254900A-Q1 Datasheet(PDF) 27 Page - Texas Instruments

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Part No. TPS254900A-Q1
Description  Automotive USB Host Charger With Short-to-VBATT Protection
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Maker  TI1 [Texas Instruments]
Homepage  http://www.ti.com
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TPS254900A-Q1 Datasheet(HTML) 27 Page - Texas Instruments

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TPS254900A-Q1
www.ti.com
SLUSCU5A – NOVEMBER 2017 – REVISED JANUARY 2018
Product Folder Links: TPS254900A-Q1
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Copyright © 2017–2018, Texas Instruments Incorporated
Typical Application (continued)
9.2.1 Design Requirements
For this design example, use the following as the input parameters.
DESIGN PARAMETER
EXAMPLE VALUE
Battery voltage, V(BAT)
18 V
Short-circuit cable
0.5 m
9.2.2 Detailed Design Procedure
To begin the design process, the designer must know the following:
The battery voltage
The short-circuit cable length
The maximum continuous output current for the charging port. The minimum current-limit setting of
TPS254900A-Q1 device must be higher than this current.
The maximum output current of the upstream dc-dc converter. The maximum current-limit setting of
TPS254900A-Q1 device must be lower than this current.
For cable compensation, the total resistance including power switch rDS(on), cable resistance, and connector
contact resistance must be specified.
9.2.2.1 Input Capacitance
Consider the following application situations when choosing the input capacitors.
For all applications, TI recommends a 0.1-µF or greater ceramic bypass capacitor between IN and GND, placed
as close as possible to the device for local noise decoupling.
During output short or hot plug-in of a capacitive load, high current flows through the TPS254900A-Q1 device
back to the upstream dc-dc converter until the TPS254900A-Q1 device responds (after t(IOS)). During this
response time, the TPS254900A-Q1 input capacitance and the dc-dc converter output capacitance source
current to keep VIN above the UVLO of the TPS254900A-Q1 device and any shared circuits. Size the input
capacitance for the expected transient conditions and keep the path between the TPS254900A-Q1 device and
the dc-dc converter short to help minimize voltage drops.
Input voltage overshoots can be caused by either of two effects. The first cause is an abrupt application of input
voltage in conjunction with input power-bus inductance and input capacitance when the IN pin is in the high-
impedance state (before turnon). Theoretically, the peak voltage is 2 times the applied voltage. The second
cause is due to the abrupt reduction of output short-circuit current when the TPS254900A-Q1 device turns off
and energy stored in the input inductance drives the input voltage high. Applications with large input inductance
(for example, a connection between the evaluation board and the bench power supply through long cables) may
require large input capacitance to prevent the voltage overshoot from exceeding the absolute-maximum voltage
of the device.
During the short-to-battery (EN = HIGH) condition, the input voltage follows the output voltage until OVP
protection is triggered (t(OV_OUT)). After the TPS254900A-Q1 device responds and turns off the power switch, the
stored energy in the input inductance can cause ringing.
Based on the three situations described, 10-µF and 0.1-µF low-ESR ceramic capacitors, placed close to the
input, are recommended.
9.2.2.2 Output Capacitance
Consider the following application situations when choosing the output capacitors.
After an output short occurs, the TPS254900A-Q1 device abruptly reduces the OUT current, and the energy
stored in the output power-bus inductance causes voltage undershoot and potentially reverse voltage as it
discharges.
Applications with large output inductance (such as from a cable) benefit from the use of a high-value output
capacitor to control the voltage undershoot.


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